Bleaching of wood pulps with thiourea dioxide



nited Sttes Ptent 3,304,534 Patented May 21, 1868 3,384,534 BLEACHING OFWOOD PULPS WITH THIOUREA DIOXIDE Robert R. Kindron, Pennington, andGeorge W. Hoi g,

Princeton, N.J., assignors to FMC Corporation, New

York, N.Y., a corporation of Delaware No Drawing. Filed Dec. 16, 1964,Ser. No. 418,911 17 Claims. (Cl. 162-71) This invention relates to thebleaching of wood pulps which contain substantial amounts ofnon-cellulosic materials, and particularly to the bleaching of thesedifiicultly bleached pulps with a reducing-bleaching agent under mildconditions.

Wood pulps are classified generally into two main classes. One class isthe so-called purified, or chemical, pulps which are prepared by pulpingmethods which involve chemical removal of much of the non-cellulose woodmaterials, such as lignins and other impurities, to leave a relativelypurified pulp composed of on the order of 80 to 100% of cellulose. Thesepulping methods involve a chemical digestion, and are typified by thekraft, sulfite and the like methods. The purified wood pulps are quitesimilar to cotton, which also is of relatively high purity, ordinarilycontaining about 80 to 95% by weight of cellulose.

The other basic class of wood pulps is prepared by methods which leavethe bulk of the non-cellulosic constituents of the wood in the pulp.These methods normally are referred to as mechanical andchemi-mechanical methods, in which the separation of the wood materialinto fibers operates primarily through mechanical attrition of the wood,either in the form of chips or as logs. The lack of a strong chemicalattack on the wood material leaves the bulk of the non-cellulosieconstituents in the fibers ultimately produced. Such fibers contain upto about 60% of cellulose, with the remaining elements in the pulp beingon the order of 40 to 60% of non-cellulose wood mate= rials.

The strictly mechanical methods for producing pulps are represented bythe so-called groundwood process, in which logs or other large pieces ofwood are ground on a grinding stone, and the refining methods in whichthe wood chips are mechanically subdivided in disc or the like refiners.Chemi-mechanical methods involve softening of the wood with aqueoussoftening agents such as sulfites, bisulfites and the like (withoutsubstantial ex= traction of non-cellulosics) preliminary to mechanicalsubdivision of the wood into fibers. Pulps produced by either of theseessentially mechanical means are referred to herein as low-cellulosewood pulps.

The low-cellulose wood pulps are particularly desired because of theirlow cost and generally satisfactory phys ical properties. Theirpreparation involves very little loss of the original wood, and methodsof producing them are generally less expensive to operate than are theso-called chemical methods. The particular utility of these pulps is inthe preparation of printing papers, newsprint, molded products,corrugated paper, boxboards and the like.

While retention of the bulk of the wood material in mechanical pulps isfavorable from an economic point of view, however, it gives rise to adark color in the pulp, and because of the large amount of non-cellulosematerial in these pulps it is difficult to bleach them to reasonablylight colors.

Generally, methods found very eifective for bleaching the comparativelypurified cellulose fibers, including cotton and the so-called chemicalpulps, have not been found effective for the low-cellulose wood pulps.One example is the method of caustic-extracting a pulp, and thenbleaching it with peroxygen chemicals. While this method is effective inbleaching kraft and sulfite pulps and cotton, the method is largelyineffective with the unpurified wood pulps having cellulosic contents onthe order of 60%.

Another example involves treatment of cotton with aqueouscaustic-formamidine sulfinate (a salt of thiourea dioxide) preliminaryto alkaline-peroxide bleaching of cotton. The caustic, preferably usedin the amount of 2 to 6% by weight in the treating solution, serves as asolubilizer and extractant for non-cellulose constituents in the cotton,and the combination of the caustic and formamidine sulfinate preparesthe cotton fibers for subsequent alkaline peroxide bleaching, impartingsome brightness itself. This method is not suitable for use with lowcellulose wood pulps. Strongly caustic solutions characteristically donot bleach these impure pulps efiectively. Other methods of treatingrelatively pure textile cotton and chemically purified wood pulpslikewise often are not useful with low-cellulose wood pulps.

It is a feature of this invention to provide a method of bleachinglow-cellulose wood pulps which markedly brightens them, yet does notextract substantial amounts of non-cellulosics from the pulps It is afurther feature of this invention to provide such a method whichoperates with an inexpensive, nonoxidizing bleaching agent. i:

It has now been found that thiourea dioxide is an ex cellent bleachingagent for low-cellulose wood pulps when it is used in very lowconcentrations and under moderate conditions not leading to damage ofthe pulp or extraction of undue amounts of non-cellulosics from thepulps. The low-cellulose wood pulp is treated for up to about 4 hours,and preferably 2 to 3 hours, at about to 300 F. and preferably 90 to 212F. and most preferably at to 180 F., in an aqueous slurry at a pulpconcentration of about 1 to 20%, and preferably about 3%, by weight at apH of about 5.5 to 8.0 with thiourea dioxide in an amount to provideabout 0.1 to 2.0% by weight of thiourea dioxide on the basis of the drypulp.

In a preferred embodiment of this invention, the bleach= ingeffectiveness of the thiourea dioxide solution is im= proved byincorporation of either or both of sodium bisulfite and sodiumtripolyphosphate or other polyphos= phate in the bleaching system. Thesodium bisulfite is used in a weight ratio to the thiourea dioxide of upto about 20 to 1, with a weight ratio of approximately 6 to 1 beingpreferred, and when this additive is used the preferred pH for the pulpslurry is about 5.5 to 7. The polyphosphate is used in an amount up to1% on the weight of the pulp, with an amount of about: 0.5% beingpreferred.

The amount of thiourea dioxide employed, and the pH of the bleachingsolution, are critical to the obtaining of optimum brightness in thepulp. Use of less than about 0.1 weight percent of thiourea dioxide onthe basis of the pulp weight, or operation ata pH of less than about5.5, results in poor bleaching. It is interesting that operation at a pHabove about 8.0 provides a substantial dropofi in bleaching effect, evenproviding substantial darken-= ing of the fiber; this would not beexpected from the prior art which is applied to the bleaching of purecellulose fibers such as cotton fibers. The upper limit on the amount ofthiourea dioxide expressed above, namely about 2.0%, is established onan economically practical basis. Effective bleaching can be achieved atsubstantially lower levels than the 2%, and operation with 0.1 to 1.0%is preferred.

The thiourea dioxide employed in this bleaching process is employedeither as the acid or as an alkali metal salt of the acid, particularlythe sodium and potassium salts. The acid has the following formula:

HN OH The thiourea dioxide sometimes is named aminoiminomethanesulfinicacid or formamidine sulfinic acid.

The pulps which are treated by the herein process are the low cellulosewood pulps, a term which is used herein to define wood pulps containingnot more than about 60% by Weight of cellulose, with the remainingmaterial being non-cellulose wood constituents. These pulps are typifiedby the mechanical and so-called chemi-mechanical pulps. The mechanicalpulps are represented by the groundwood pulps and the refiner pulps,produced respectively by grinding wood logs on grinding stones, and bypassing wood chips through a disc or other refiner to mechanicallysubdivide the wood into fibers. The chemimechanical pulps are providedby softening the wood prior to mechanical subdivision of the fibers. Thesoftening agents may be aqueous sulfite, bisulfite or other commonsoftening agents, used in a fashion to avoid extraction of more than onthe order of to 20% of the non-cellulosics in the wood during thesoftening and pulping stages. These low-cellulose wood pulps areproduced from a variety of wood sources, for example, spruce, fir,hemlock, pine, cottonwood, aspen and poplar.

The low-cellulose wood pulps are treated in the present process in theform of an aqueous slurry having a concentration of about 1 to 20% byweight, and preferably of about 3% by weight. Treatment of slurriescontaining substantially less than 1% of the pulp is uneconomical,requiring excessive equipment investment, while treatment of pulpshaving concentrations greater than about 20% is difiicult because of themechanical problems as sociated with providing good mixing of thetreating chemicals with the pulp. The process is a reducingbleachingprocess and accordingly is carried out under non-oxidizing conditions.In small scale operation it is sometimes desirable to establish anon-oxidizing, inert atmosphere over the pulp, e.g. by creating anitrogen or other inert atmosphere in the bleaching chamber. In normalcommercial scale operation this is unnecessary as normal plant operationconditions involving large batches are not such as to lead to oxidation.

The bleaching process of this invention is carried out at a temperatureof about 90 to 300 F., preferably about 90 to 212 F., and mostpreferably about 160 to 180 F. These temperatures are such as to assurere= tention of good pulp properties, while at the same time providingeffective bleaching in a reasonable time. Operation at temperaturesbelow about 90 F. provides for too low brightness increases and for toolong bleaching times, while operation at above about 300 F. isunnecessary and uneconomical. Normally the operation is carried out nohigher than 212 F. except in such processes as the high consistencyrefiner bleach process.

The herein bleaching treatment is carried out for a time up to about 4hours, and preferably about 2 to 3 hours. Operation for less than onehour normally does not provide the desired degree of brightnessimprovement, whereas operation for more than about 4 hours isunnecessary.

The pH of the aqueous phase of the pulp slurry is about 5.5 to 8.0. Toprovide this pH when the thiourea dioxide is used without sodiumbisulfite, about 0 to 0.2% of an alkali, for example sodium hydroxide,potassium hydroxide, sodium carbonate, potassium carbonate or the like,is provided in the aqueous phase of the pulp slurry. More alkali,sometimes as much as 0.4% of the aqueous phase of the slurry may berequired when sodium bisulfite is present, since this material reactswith and neutralizes some of the added sodium hydroxide. The

actual amount of alkali within this range employed is based on theacidity or alkalinity of the pulp being treated and its concentration inthe pulp slurry, and is the amount required to provide the desired pHwithin the range of 5.5 to 8.0 pH.

Addition of sodium bisulfite and/or sodium tripolyphosphate or otherpolyphosphates materially improves the bleaching effect of the thioureadioxide solution. The sodium bisulfite is used in amounts expressed interms of the weight ratio of sodium bisulfite to thiourea dioxide, up toabout 20 to 1. An amount of sodium bisulfite providing a weight ratio ofabout 6 parts of the bisulfite to 1 part of the thiourea dioxide ispreferred. Use of more than the expressed upper amount of. 20 to 1 ofbisulfite to thiourea dioxide is wasteful, not providing added bleachingadvantage. The polyphosphate is employed in amounts up to 1% by weight.based on the dry weight of pulp, with 05% being preferred. Typicaluseful polyphosphates are sodium tripolyphosphate, tetrasodiumpyrophosphate, sodium hexametaphosphate and the like.

Use of the sodium bisulfite is best carried out at a pulp slurry pH nearthe lower end of the herein range of 5.5 to 8. When bisulfite isemployed addition of sufficient sodium hydroxide to raise the pH toupwards of 8.0 is wasteful of both sodium hydroxide and the bisulfitesince these chemicals are partially consumed in a neutralizationreaction. Accordingly, it is preferred to operate at a pH of about 5.5to 7 when using sodium bisulfite.

The pH of the pulp is normally adjusted upwardly by addition of sodiumhydroxide. However, equivalent alkalies such as sodium carbonate,potassium hydroxide, potassium carbonate and the like may be employed inplace of the sodium hydroxide to adjust the pH to the desired level.

The bleaching process preferably is carried out by slurrying the pulp inwater at the desired concentration, and adding the thiourea dioxide andif necessary the sodium hydroxide or other alkali, together with anydesired ad= ditives such as the sodium bisulfite or the polyphosphate,and the like, to the pulp slurry either separately or to gether. Mildagitation is provided initially to provide even distribution ofbleaching chemicals. The temperature is raised to the desired level byexternal heating means, by passing steam through the pulp slurry or thelike. Follow= ing the bleaching treatment, if desired although notnecessarily, the pulp is washed with water and either dried or useddirectly in producing end products.

The following examples are presented by way of illus-- tration of thisinvention and are not to be considered as limiting the scope thereof inany Way.

Example 1 A southern pine groundwood having an unbleached GE brightnessof 61.0% was slurred with boiling dilution water in a slow-speed paddlemixer to a concentration at which, following addition of bleachingchemicals, the final concentration became 3% by weight of pulp. Thisconcentration of 3% in the final slurry was used in the followingexamples, with the exception of Example 8, in which concentration Wasvaried. The pulp slurry was then transferred to a four-liter screw toppolyethylene container, and the air in the container was replaced withnitrogen gas, after which the following chemicals, in the indicatedamounts and dissolved in water, were added and the cap placed on thebottle. Thereafter the bottle containing the pulp slurry and addedchemicals was shale en, and 50 ml. of pulp slurry was removed for pHdetermination. The bottle was then placed in a water bath held at F. toprovide the desired bleaching temperature, and left there for 4 hours,Following the treatment the pulp was withdrawn, and its brightnessdetermined on a Gardner refieetometer, with the brightness values beingcorrected to GE standards.

Thiourea dioxide was employed in the amount of 0.75 weight percent.based on the dry weight of pulp,

sodium hydroxide was present in the water phase of the pulp slurry at aconcentration of 0.006 weight percent to provide a pH in the pulp of7.1.

The brightness of the bleach pulp was 71.2%, a 10.2 point increase overthe initial GE brightness.

Example 2.CornpariSOn with sodium hydrosulfitea known reducing agentbleach The procedure employed in this example was the same as thatfollowed in Example 1. In this case a southern pine groundwood having anunbleached brightness of 61.0% was treated in a series of tests, eachproviding for a 4 hour treatment at 160 F., with chemicals indicated inthe following table (Table 1) in the indicated amounts.

1 Percent by weight based on the dry weight of the pulp. 2 Sodiumhydrosulfite.

3 Thiourea dioxide.

4 Sodium bisulfite.

Example 3 This example demonstrates the effect of the amount of thioureadioxide employed in the process of this invention. A southern pinegroundwood having a GE brightness of 61.0% was treated according to theprocedure of Example l, again over a period of 4 hours at 160 F. The pHof the slurry of pulp was 7.1 to 7.2, which condition was achieved byproviding sodium hydroxide in the aqueous phase of the slurry at aconcentration of 0.005 to 0.006% by weight. The increases in brightnessprovided by the treatments are shown in column 2 of table 2 whichfollows.

TABLE 2 TDO percent Brightness 0.17. Pulp ln rease Basis Points Example4 This example demonstrates the effect of sodium bisulfite when it isemployed together with the thiourea dioxide bleaching agent. A southernpine groundwood having an unbleached GE brightness of 61.0% was treatedas described in Example 1. The pH of the system was 6.6 to 7.2, and wasobtained by providing sodium hydroxide at a concentration of 0.003 to0.008% by weight of the aqueous portion of the pulp slurry. Thioureadioxide was employed in the amount of 0.25% on the dry weight of thepulp, together with amounts of sodium bisulfite providing the weightratios to the amount of thiourea dioxide indicated in the first columnof Table 3 which follows. The brightness increases are shown in the lastcolumn of this table.

6 Example 5 This example demonstrates the effectiveness of a typicalpolyphosphate, sodium tripolyphosphate, in improving the bleachingeffect of thiourea dioxide. A northern spruce/ fir groundwood having anunbleached GE brightness of. 59.7% was treated by the method describedin Example 1. The treating chemicals were thiourea dioxide employed inthe amount of 0.5% on the basis of the dry weight of the pulp, andsodium tripolyphosphate, again on the basis of the dry weight of thepulp, in the amounts indicated in the first column of Table 4 whichfollows. The pH of the system was 7.7 to 7.8, provided by employingsodium hydroxide in the aqueous system at a concentration of about 0.006to 0.009% by weight. The brightness increases obtained after 4 hourtreatments at 160 F. are shown in the last column of Table 4.

TABLE 4 Wei ht Parent Brightness STPP, pereznt pare nt pH N .0 H Inrease 'lDO solu. b.=sis Points 1 Sodium tripolyphosphate.

Example 6 The effect of the pH on the bleaching effect of the thioureadioxide is demonstrated in this example. A southern pine groundwoodhaving an unbleached GE brightness of 61.0% was treated as described inExample 1. Thiourea dioxide was employed in the amount of 0.5% by weightbased on the dry weight of the pulp, and the concentrations of sodiumhydroxide in the aqueous system shown in Table 5 which follows, wereemployed to vary the pH. Brightness increases are shown in the third.column in Table 5.

The effect of temperature on the bleaching process is demonstrated inthis example. A southern pine groundwood having an unbleached GEbrightness of 61.0% was treated by the method shown in Example 1.Five-tenths percent of thiourea dioxide on the dry weight of the pulpswas employed in these experiments, together with an amount of sodiumbisulfite to provide a 6:1 weight ratio thereof to the thiourea dioxide.The pH was 6.6 to 6.7 (sodium hydroxide being present in the aqueousphase of the slurry at a concentration of 0.016 weight percent). Samplesof this pulp were treated for 4 hour periods at the temperaturesindicated in column 1 of Table 6 which follows, with the results shownin column 2 as brightness increases in GE percentage points.

TABLE 6 Weight Weight per 'ent Patio Brightness TDO TDO/ Increase 0. 56/ l 4. 8 0. 5 (ill 7. 8 0. 5 6/1 10. 2 0. 5 (5/1 10.8

Example 8 The effect of the slurry concentration of the pulp on thebleaching process is demonstrated in this example. A southern pinegroundwood pulp having an unbleached GE brightness of 61.0% was preparedas aqueous slurries whose concentrations following addition of bleachingchemicals were those expressed in Table 7 which follows. The slurrieswere treated in accordance with the procedure described in Example 1.Five-tenths percent by 61A method of bleaching mechanical andchemimechanical low-cellulose wood pulps containing up to 60% cellulose,in which said low-cellulose wood pulp is treated at a temperature of 90to 300 F. for up to 4 weight of thiourea dioxide on the dry weight ofthe pulp, 5 hours in an aqueous pulp slurry having a pH of 5.5 to 7.0and sodium bisulfite in an amount to provide a 6:1 weight and containing1 to 20% by weight of said pulp, thiourea ratio thereof to the thioureadioxide, were employed. A dioxide in an amount to provide 0.1 to 2.0% ofthiourea pH of 6.3 to 6.7 was provided by employing sodium hydioxide onthe weight of the pulp and sodium bisulfite in droxide in the aqueousphase of the slurries at the conan amount up to 20 times the weight ofsaid thiourea dicentmtions set forth in Table 7. The brightnessincreases 10 oxide. which were achieved are shown in this Table 7. 7.The method of claim 6 in which the sodium bisulfite TABLE 7 Weightpercent. Pul Weight Ratio NaOli weight Brightness Concentration TDONmszo rno percent in Increase Aqueous Phase Points It can be seen fromthe above examples that thiourea dioxide is a very eifective bleachingagent for the lowcellulose wood pulps when employed in the amount of0.25% or more by weight based on the dry weight of pulp at a pH of 5.5to 8.0. It is shown also that the effectiveness of this process isdependent upon careful observance of certain critical limitations onamounts of ingredients, pH, temperatures and the like. It is also shownthat thiourea dioxide is better than one of the best prior art reducingbleaches, sodium hydrosulfite, in its bleaching efiectiveness. Theseexcellent bleaching results are con= firmed with other low-cellulosewood pulps, and the method of this invention provides a very usefulmeans for bleaching these diificultly bleached pulps.

Pursuant to the requirements of the patent statutes, the principle ofthis invention has been explained and exemplified in a manner so that itcan be readily practiced by those skilled in the art, suchexemplification including what is considered to represent the bestembodiment of the invention. However, it should be clearly understoodthat within the scope of the appended claims, the invention may bepracticed by those skilled in the art, and having the benefit of thisdisclosure, otherwise than as specifically described and exemplifiedherein.

What is claimed is:

1. A method of bleaching mechanical and chemimechanical low-cellulosewood pulps containing up to 60% cellulose, in which said low-cellulosewood pulp is treated at a temperature of 90 to 300 F. for up to 4 hoursin an aqueous pulp slurry having a pH of 5.5 to 8.0 and containing 1 toby weight of said pulp and thiourea dioxide in an amount to provide 0.1to 2.0% of the thiourea dioxide on the weight of the pulp.

2. The method of claim 1 in which 0.1 to 1.0% of thiourea dioxide on theweight of pulp is used.

3. The method of claim 1 in which the temperature of the bleachingtreatment is 90 to 212 F., and the time of said treatment is 2 to 3hours.

4. The method of claim 1 in which the temperature of the bleachingtreatment is 160 to 180 F., and the time of said treatment is 2 to 3hours.

5'. The method of claim 1 in which the pulp concentration in the slurryis about 3%.

is used in the amount of about 6 times the weight of thiourea dioxideemployed.

8. The method of claim 6 in which 0.1 to 1.0% of thiourea dioxide on theweight of the pulp is used.

0. The method of claim 6 in which the temperature of the bleachingtreatment is to 212 F., and the time of said treatment is 2 to 3 hours.

10. The method of claim 6 in which the temperature of the bleachingtreatment is to F., and the time of said treatment is 2 to 3 hours.

11. The method of claim 6 in which the pulp concentration in the slurryis about 3%.

12. A method of bleaching mechanical and chemimechanical low-cellulosewood pulps containing up to 60% cellulose, in which said low-cellulosewood pulp is treated at a temperature of 90 to 300 F. for up to 4 hoursin an aqueous pulp slurry having a pH of 5.5 to 8.0 and containing 1 to20% by weight of said pulp, thiourea dioxide in an amount to provide 0.1to 2% of thiourea dioxide on the weight of the pulp and a polyphosphatein an amount of up to 1% of the weight of the pulp.

13. The method of claim 12 in which the polyphos= phate is sodiumtripolyphosphate and said sodium tripolyphosphate is employed in anamount of about 0.5% of the weight of the pulp.

14. The method of claim 12 in which 0.1 to 1.0% of thiourea dioxide onthe weight of the pulp is used.

15. The method of claim 12 in which the temperature of the bleachingtreatment is 90 to 212 F., and the time of said treatment is 2 to 3hours.

16. The method of claim 12 in which the temperature of the bleachingtreatment is 160 to 180 F., andthe time of said treatment is 2 to 3hours.

17. The method of claim 12 in which. the pulp concentration in theslurry is about 3%.

References Cited UNITED STATES PATENTS 2,860,944 11/1958 Young 16276XDONALL H. SYLVESTER, Primary Examiner. H. R. CAINE, Examiner.

1. A METHOD OF BLEACHING MECHANICAL AND CHEMIMECHANICAL LOW-CELLULOSEWOOD PULPS CONTAINING UP TO 60% CELLULOSE, IN WHICH SAID LOW-CELLULOSEWOOD PULP IS TREATED AT A TEMPERATURE OF 90* TO 300* F. FOR UP TO 4HOURS IN AN AQUEOUS PULP SLURRY HAVING A PH OF 5.5 TO 8.0 AND CONTAINING1 TO 20% BY WEIGHT OF SAID PULP AND THIOUREA DIOXIDE IN AN AMOUNT TOPROVIDE 0.1 TO 20% OF THE THIOUTEA DIOXIDE ON THE WEIGHT OF THE PULP.